The improvement of low fuel cost is required for a rubber composition for a breaker topping, while keeping rigidity, elongation at break and peel strength at high level.
Usually, when sulfur is used as a vulcanizing agent for a diene rubber used for a rubber composition for a breaker topping and vulcanization at high temperature is carried out, polysulfide bond linking between crosslinking points is cleaved and reversion (vulcanization return) occurs. When the reversion occurs, the properties of vulcanized articles are lowered and it has been difficult to obtain objective physical properties.
As a method of suppressing the reversion, a method of reducing the content of sulfur is known. However, in this case, there have been problems that time is required for vulcanization speed, the productivity of a vulcanized rubber is lowered and the adhesion with a steel cord is occasionally lowered. Further, as a method of suppressing the reversion, a method of including synthetic rubbers such as a styrene-butadiene rubber (SBR) and a butadiene rubber (BR) is known. However, in this case, there have been problems that fracture strength is lowered and durability is lowered.
Accordingly, it is required for the rubber composition for a breaker topping that the reversion is suppressed without lowering adhesion property and fracture strength. On the other hand, request for low fuel cost has been recently enhanced and the breaker topping is also desired to further improve low fuel cost.
As a method of improving the low fuel cost, a method of reducing loss tangent (tan δ) by reducing the content of fillers such as carbon black and silica has been known. However, in this case, there has been problems that complex elastic modulus (E*) is lowered and rigidity is lowered.
Consequently, it is desired for the rubber composition for a breaker topping that tan δ is reduced without lowering rigidity.
As described above, it is desired for the rubber composition for a breaker topping to suppress the reversion and reduced tan δ without lowering adhesion property, fracture strength and rigidity. But nothing is really realized yet.
Japanese Unexamined Patent Publication No. 2007-277289 discloses a rubber composition for a breaker topping including rubber components containing a butadiene rubber containing 2.5 to 20% by weight of 1,2-syndiotactic polybutadiene crystals and a modified butadiene rubber and/or a modified styrene-butadiene rubber, in order to provide a rubber composition for a breaker topping capable of improving low fuel cost while keeping rigidity, elongation at break and peel strength at high level. However, the improvement of strength at break, in particular, elongation at break (EB) and low heat build-up (low tan δ), is susceptible to improvement.